Method and apparatus for producing welding power

ABSTRACT

A method and apparatus for providing submerged arc welding power is disclosed. The power supply is AC/DC, and may be controlled in either a CV or a CC mode. The power supply includes a cycloconverter that provides a single phase output and receives a three phase input. A controller includes a PI current regulator for operation in the CC mode. The controller also includes a PI voltage regulator. When the CV mode is selected, the voltage and current regulators are cascaded such that the output of the voltage regulator is the set point input to the current regulator.

This is a continuation of application Ser. No. 08/872,845 filed on Jun.11, 1997 now U.S Pat. No. 6,114,655.

FIELD OF THE INVENTION

The present invention relates generally to the art of power supplies forelectric arc welding and, more particularly, to power supplies forelectric arc welding that provide a CC/CV output that may be either ACor DC, and that are particularly well adapted for submerged arc welding.

BACKGROUND OF THE INVENTION

Submerged arc welding (also called SAW or sub arc) is a type of arcwelding where the arc is not visible. Sub arc welding producescoalescence of metals by heating them with an arc between a bare metalelectrode and the work piece. The arc and molten metal are submerged ina blanket of granular fusible flux on the work piece. Filler metal isprovided by the electrode (or from a supplemental source such as awelding rod or metal granules). The arc is covered by the flux.

Many sub arc applications are automatic welding applications whereeither the work piece is moved under the weld head or the weld head ismoved over the stationary work piece. Such automatic systems includewire feeders and are well known in the art. Wire feeders used in sub arcwelding may be either constant speed or variable speed. Constant speedwire feeders are typically used with CV power supplies, and variablespeed wire feeders may be used with CC power supplies. Each type of wirefeeder has advantages and disadvantages. Preferably, a welding powersupply should be useable with a constant speed wire feeder, or useablewith either type of wire feeder.

Early automatic sub arc welding applications provided a DC output andused power sources with drooping V-A characteristics and voltagefollowing wire electrode feeders. Subsequently, constant voltage (CV) DCsources were introduced to the process and linked to constant speed wireelectrode feeders. However, magnetic fields generated by the DC arccurrent and surrounding the arc and the field associated with the groundcurrents react with each other in an unpredictable manner, causing thearc to move as if the arc were being “blown” to one side. This isreferred to as arc blow. This effect is most objectionable in deep grovewelds where erratic movement of the arc disturbs proper formation andplacement of the weld puddle. Arc blow becomes a more severe problem asthe amperage increases, because magnetic fields correspondinglyincrease.

Arc blow is less of a problem when using an AC power supply (becausethere is not a DC arc current). However, a sinusoidal output does notalways perform well in sub arc welding processes because the sinusoidalwave exhibits a slow zero crossover which may result in arcrectification.

Square wave welding power sources attempt to use the advantages ofsinusoidal AC welding, but with a rapid zero crossing to avoid arcrectification. One known square wave welding power supply is describedin U.S. Pat. No. 4,038,515 issued to Risberg. This power supply providesfor a square wave AC welding output. The Risberg design provides aconstant current (CC) output and thus cannot be used with a constantspeed wire feeder. The output of this power supply is at a frequencyequal to the input frequency.

Another prior art sub arc welding power supply is described in U.S. Pat.No. 4,322,602 which was issued to Grist, and was owned by the assigneeof the present invention. Grist describes an AC constant potential (CV)power source which may be used for sub arc welding. The output of Gristis an AC/CV output having a frequency equal to the input frequency, andhaving a fast zero crossing. This power supply is used with a constantspeed wire feeder.

A TIG (Tungsten inert gas) welding power supply is described in U.S.Pat. No. 5,340,963, which is also owned by the assignee of the presentinvention, and is hereby incorporated by reference. U.S. Pat. No.5,340,963 shows an AC power source for welding which receives a threephase input and provides a single phase AC output, having relativelyfast zero crossings, at a frequency 1.5 times the input frequency. Thisis a type of step-up cycloconverter. However, this prior art does notteach a CV mode of operation, nor a CC controller. This prior art can beoperated in a DC mode, but only operates on half of the sinusoidal input(thus, the SCRs and secondary windings must be able to handle twice thecurrent, relative to the current capacity needed if the entire inputwere used). This can be costly and add weight and size to the machine.

A “step-up cycloconverter”, as used herein, is a cycloconverter havingan output frequency greater than the input frequency. It receives an ACinput at a given frequency and provides an AC output at a higherfrequency. This conversion is obtained by phase control or without usingswitches that are forced off, such as force commutated SCRs, IGBTs orFETs. Thus, a rectifier followed by an inverter or buck/boost converteris not a cycloconverter. The applicants have learned that sub arcwelding performed at a frequency greater than the input line frequency(50 or 60 Hz) will provide a better weld. Power sources that provide anoutput at greater that than 60 Hz are known and are generally invertersor other converters. However inverter based converters require the useof expensive switches that may be turned off, such as IGBT's. This isparticularly true in applications such as sub arc welding where thecurrent desired may exceed 1000 amps. Accordingly, inverter based powersupplies for use in sub arc welding may be expensive and not practical.

Additionally, it is desirable to provide flexibility in a welding powersupply so that it may be used for a variety of applications. Forexample, it is desirable to provide a welding power supply that providesan AC or DC output. Also, it is desirable to provide a welding powersupply that provides either a CV or a constant current (CC) output, thatmay be used with a constant or variable speed wire feeder. Inverterbased welding power supplies may be AC/DC and CC/CV, but as describedabove, they may be expensive, and not appropriate for sub arcapplications.

Accordingly, it is desirable to provide a welding power supply that issuitable for sub arc welding that maybe operated either a CC, or a CVmode. Also, such a power supply preferably be operable to provide anoutput having a frequency greater than the input line frequency, when inthe AC mode, but not require the use of IGBTs or other switches that maybe turned off.

SUMMARY OF THE PRESENT INVENTION

According to a first aspect of the invention a submerged arc weldingpower supply includes a step-up cycloconverter that has a control input.A controller is coupled to the control input.

The controller has a feedback input in one embodiment. The controllerreceives a signal indicative of an output parameter on the feedbackinput, amd controls the output of the cycloconverter in response to thefeedback input. Other alternatives include the output parameter beingoutput voltage or current.

The controller includes at least two feedback inputs in anotheralternative. Signals indicative of the output voltage and output currentare received on the feedback inputs. The controller controls the outputof the cycloconverter in response to a user selectable one of outputcurrent and output voltage.

The controller includes a current regulator and a voltage regulator inone embodiment. If the users selects a CV mode of operation then thevoltage and current regulators are cascaded such that the cycloconverteris controlled in response to the output voltage.

The output is selectable between AC and DC in one embodiment.

Another aspect of the invention is an arc welding power supply thatincludes a step-up cycloconverter. A feedback circuit provides a signalindicative of the output voltage. A controller is coupled to thecycloconverter the feedback circuit. The controller controls the outputof the cycloconverter in response to the output voltage to provide aconstant voltage output.

A current feedback circuit is also coupled to the controller in analternative embodiment. The controller controls the output of thecycloconverter in response to a user selectable one of output currentand output voltage.

The controller includes voltage and current regulators in oneembodiment, If the user chooses to operate in a CV mode then the voltageand current regulators are cascaded such that the cycloconverter iscontrolled in response to the output voltage.

Yet another aspect of the invention is a submerged arc welding powersupply that has a three phase input at an input frequency. A converterreceives the input and provides a single phase output having an outputfrequency that is greater then the input frequency.

The output frequency is 1.5 times the input frequency in one embodiment.

Another embodiment provides for the controller to have a feedback inputthat receives a signal indicative of an output parameter. The controllercontrols the output of the converter in response to the parameter.

The controller includes a PI current regulator and the output parameteris output current in one embodiment. The controller includes a PIvoltage regulator and the output parameter is output voltage in anotherembodiment.

The user can select between a CC mode and a CV mode in anotherembodiment. If the user selects CV mode then the voltage and currentregulators are cascaded.

Another aspect of the invention is a method of producing power forsubmerged arc welding by providing input power to a cycloconverter. Thefrequency of the power is stepped up using the cycloconverter. Theoutput of the cycloconverter is controlled in response to at least oneoutput parameter.

The cycloconverter is controlled in response to output voltage (CVoperation) in one embodiment, and in response to output current (CCoperation) in another embodiment. The user may select one of CV or CCoperation in another embodiment.

One alternative provides for controlling the cycloconverter by producinga PI voltage error signal and providing the voltage error signal as aset point to a PI current regulator. A PI current error signal isprovided and the cycloconverter is controlled in response to the currenterror output.

Another aspect of the invention is producing arc welding power bycycloconverting a three phase input. An output voltage feedback signalis produced and provided to a controller. The output of thecycloconverter is controlled in response to the output voltage toprovide a constant voltage output.

The method includes producing a PI voltage error signal and providingthe error signal as a set point to a PI current regulator in analternative embodiment. A PI current error is produced, and thecycloconverter is controlled in response to the current error output.

Another aspect of the invention is a method of producing submerged arcwelding power comprising by receiving a three phase input power at aninput frequency and converting the input power to single phase outputpower having an output frequency that is greater then the inputfrequency.

The output frequency is 1.5 times the input frequency in one embodiment.

Other principal features and advantages of the invention will becomeapparent to those skilled in the art upon review of the followingdrawings, the detailed description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of the power supply of the preferredembodiment, with jumpers showing the connections for either an AC or DCmode;

FIG. 2 is a circuit diagram of the power supply of the preferredembodiment configured in the AC mode;

FIG. 2A is a circuit diagram of the power supply of the preferredembodiment configured in the AC mode, wherein most of the non-powercomponents are not shown;

FIG. 3 is a circuit diagram of the power supply of the preferredembodiment configured in the DC mode;

FIG. 3A is a circuit diagram of the power supply of the preferredembodiment configured in the DC mode, wherein most of the non-powercomponents are not shown;

FIG. 4 is a circuit diagram of a current regulator used in the preferredembodiment; and

FIG. 5 is a circuit diagram of a voltage regulator used in the preferredembodiment.

Before explaining at least one embodiment of the invention in detail itis to be understood that the invention is not limited in its applicationto the details of construction and the arrangement of the components setforth in the following description or illustrated in the drawings. Theinvention is capable of other embodiments or of being practiced orcarried out in various ways. Also, it is to be understood that thephraseology and terminology employed herein is for the purpose ofdescription and should not be regarded as limiting. Like referencenumerals are used to indicate like components.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the present invention will be illustrated with reference to aparticular power circuit and controller, it should be understood at theoutset that the invention may include the addition of other components,removal of components, or the substitution for components. The preferredexample, including component values, is not limiting, rather it asexemplary. One skilled in the art should be able to use other componentsand component values to implement this invention.

A circuit diagram of a AC/DC CC/CV welding power supply configured inaccordance with the present invention is shown in FIG. 1. This weldingpower supply is a submerged arc welding power supply and should becapable of operating in a CV mode. A three phase sinusoidal input isreceived on a plurality of inputs Φ1, Φ2, and Φ3. A plurality ofcapacitors C1, C2, and C3 are provided between the inputs and ground toact as high frequency filters. A of contactor having a plurality ofcontacts W is used to connect the input power to a transformer T1.

The primary transformer T1 is connected in a delta connection, althougha wye connection could be used. A delta connection is used in thepreferred embodiment because it is designed using wires with a lessercurrent carrying capacity, but with greater number of turns.

The secondaries of transformer T1 are split, center tapped secondaries.“Split, center tapped secondary,” as used herein, is a center-tappedsecondary, wherein other circuitry (SCRs e.g.) may be disposed betweeneach set of windings and the center tap. The turns ratio in thepreferred embodiment is about 6.9:1 so that 460 volts on Φ1, Φ2, and Φ3produces about 70 volts open circuit.

The power supply of FIG. 1 may be used to provide either an AC or DC(AC/DC) output. The user selects between an AC and DC output byappropriately connecting a plurality of jumpers. A box 102 shows theposition of the jumpers for the AC and DC modes of operation.

A plurality of SCR's, SCR1-SCR6, are connected to the secondarywindings. Each SCR is provided with a capacitor (C11-C16) and a resistor(R11-R16) to act as a snubber. The configuration of SCRs 1-6 dependsupon the mode (AC or DC) of operation. A controller 103 provides thegating signals to SCR's 1-6 so that they conduct in a manner such asthat described in the AC mode of U.S. Pat. No. 5,340,963.

An inductor L1 (400 microhenry each winding), which is a center tapinductor, helps provide a smooth output and assists in rapid zerocrossing when the power supply is operated in the AC mode. L1 isconfigured so that current will flow through the inductor in the sameflux creating direction regardless of the direction of current in theprimary (and associated secondary) winding. Each leg of inductor L1 iscomprised of two magnetically parallel windings made of aluminum andmounted on opposite legs of a U core to help carry the high current loadin the preferred embodiment. Inductor L1 will be connected in one of twoways (as shown in FIGS. 2 and 3), depending upon the mode of operationselected (AC or DC).

A pair of resistors R1 and R2 (20 ohms) are provided to help SCRs 1-6latch ON under no load condition. A hall device HD1 is provided to sensethe current output and provide it to controller 103 on an input RC2-1.Inputs RC1-1 and RC1-4 provide power to hall device HD1.

A pair of capacitors C9 and C10 are provided to an output electrode Eand a workpiece W. Capacitors C9 and C10 have a capacitance of 0.1 μFand are provided to act as high frequency filters.

A pair of resistors R3 and R4 (200 ohms) are used to sense the outputvoltage provided to electrode E and work W, and are provided tocontroller 103 on inputs RC2-4 and RC2-5. Controller 103 includes acurrent regulator for operation in the constant current mode which usesthe current feedback from hall device HD1. In the preferred embodiment(described in more detail below with reference to FIG. 4), a typicalanalog PI current regulator is provided. Controller 103 also includes avoltage regulator for operation in the CV mode which uses the voltagefeedback from resistors R3 and R4. In the preferred embodiment(described in more detail below with reference to FIG. 5), a typicalanalog PI voltage regulator is provided.

The output of the power circuit may be controlled by controlling thephase angle at which the SCR's are fired. To increase the output of thepower circuit the SCR's are phased forward so that they fire earlier ineach cycle. Conversely, to decrease the output, the SCR's are phasedback so that they fire later. The nominal conduction time for each SCRis 120 degrees (of the input cycle) so that the output frequency is 1.5times the input frequency.

Controller 103 receives an input command ISET indicative of a userselected output current. ISET is obtained from a front panelpotentiometer (or a remote or other input device) of the welding powersupply when it is operated in the CC mode. Controller 103 compares ISETto the sensed current and controls the firing angle of SCRs 1-6accordingly. A variable speed wire feeder is used in the CC mode in thepreferred embodiment.

Controller 103 also includes an input VSET indicative of a user selectedoutput voltage for operation in the CV mode. VSET allows the user toselect a Constance voltage output. The PI voltage regulator ofcontroller 103 has an output dependent upon the difference between VSETand the output voltage received via resistors R3 and R4. The output ofthe voltage regulator is provided as the set point input, ISET, (insteadof the user selected output current) to the current regulator. Thecurrent and voltage regulators are thus cascaded such that a CV outputis obtained. A constant speed wire feeder is used in the CV mode in thepreferred embodiment.

The power supply of FIG. 1, having the jumpers connected in the AC mode,is shown in FIG. 2. A plurality of terminals 47, 87 and 88, areconnected to the undotted end of one leg of each secondary ontransformer T1. These legs of each secondary are connected to, on thedotted end, SCRs 1, 3 and 5. SCRs 1, 3 and 5 are configured to allowcurrent flow (when on) from the dotted end of the secondary to the SCR.SCRs 1, 3 and 5 are also each connected to a terminal 50 (through aplurality of terminals 90, 91 and 92). Terminal 50 is also connected toworkpiece W. The undotted end of these secondaries is connected to anend of inductor Li. The center tap of inductor L1 is connected to theelectrode.

The other halves of the secondaries have their dotted ends connectedtogether and also connected to the other end of inductor L1. Theundotted ends of these secondaries are connected to SCRs 2, 4 and 6,which allow current to flow into the undotted ends of the secondaries(when on). SCRs 2, 4 and 6 are also connected to the workpiece.

The current paths will now be described with respect to secondaries S1Aand S1B, which are associated with primary P1. When SCR6 is on, currentflows through SCR6 to the undotted end of secondary S1A, then from thedotted end of secondary S1A through inductor L1 to electrode E, throughthe arc to workpiece W, and back to SCR6. Similar current paths existwith SCRs 2 and 4. When SCR5 is on, current flows from the dotted end ofsecondary S1B through SCR5, to workpiece W, through the arc to electrodeE, through L1, and back to the undotted side of the secondary. Similarcurrent paths exist with SCRs 1 and 3.

FIG. 2A is a simplified circuit diagram showing the power supply ofFIGS. 1 and 2 connected in the AC mode. However, FIG. 2A primarily showsthe secondary side power components on the configured in the AC mode,and omits snubbers etc., and the jumpers relating to the DC mode.

FIG. 2A shows that, in the AC mode, the power circuit of the presentinvention, is configured much like the power circuit of U.S. Pat. No.5,340,963. Specifically, when the SCR's are fired in a sequence of SCR6,SCR3, SCR2 SCR5, SCR4, SCR1, and each SCR is conducts for at most 120degrees before the zero crossing, an output signal having a frequency of1.5 times the input line frequency is created. However, according to thepresent invention, the power circuit may be operated in a CC or CV mode.Thus, it may be seen that a step up cycloconverter is provided whichoperates in a CV or a CC mode.

Because the output is provided through center tap conductor L1 so thatthe current is always flowing in the same flux creating direction in L1,regardless of the direction of the output current flow, inductor L1smooths the welding current and assists in a rapid zero crossing.

The power source as configured in a DC mode is shown in FIG. 3. SCR's 1,3, and 5 are connected to bus bar 101 while SCR's 2, 4, and 6 areconnected to the work piece in the DC mode. The SCR's are fired in thesame sequence as that for the AC mode. However, in this configurationthe output will be a DC output. Inductor L1 is used to provide asmoother welding output.

Current flows from the dotted side of secondary S1A through inductor L1,through the arc, through SCR6, and back to secondary SiA. Similarcurrent paths exist through SCRs 2 and 4. Another current path is fromthe dotted end of secondary S1B, through SCR5, through inductor L1 (inthe same flux-creating direction sa the other current path), through thearc and back to secondary S1B. Similar current paths exist through SCRs1 and 3.

FIG. 3A is a simplified schematic of the power circuit of FIGS. 1 and 3where the primary windings and certain associated circuitry likessnubbers are not shown. Thus, a power circuit that operates in AC or DCmode, with a controller that can provide a CC or CV output has beendescribed. The output is preferably used in sub arc welding.

Controller 103 allows the user to select between a CV mode and a CC modeof operation. Selection is preferably made using a toggle switch on thefront panel of the power supply (or using a remote). Two regulators areprovided: a current regulator and a voltage regulator. When CC operationis selected, the current regulator is used to control the output, andthe voltage regulator is not used. When CV operation is selected, theoutput of the voltage regulator is used as the set point for the currentregulator. The control is then based on the current regulator output.This cascading arrangement allows the user to select a CV output, andstill use the current regulator to control the SCR firing.

The current regulator is a proportional integral (PI) current regulator,in the preferred embodiment, and is shown in FIG. 4. The currentfeedback signal is provided on RC2-1 from hall device HD1 (see FIG. 1).The current feedback signal is provided across a resistor R31 (5 ohms)and a capacitor C21 (0.1 μF) which filter the current feedback signal.

The filtered signal is provided to resistors R33 (475 ohms), R34 (511ohms), R35 (10K ohms), R36 (200K ohms) and an op amp A3B. Op amp A3Bscales the current feedback signal and provides it through a resistorR25 (39.2K ohms) to the integrating portion of the circuit.

The current reference signal (ISET) is provided on pin RC2-2. Thecurrent reference signal is, in the preferred embodiment, derived from apotentiometer on the front panel when the power supply is operated inthe CC mode. The current reference input varies between 0 and 10 volts.The current reference input on pin RC2-2 is obtained from the output ofthe voltage regulator (described below) when the power supply isoperated in the CV mode.

The current reference signal (ISET) is provided through an inductor L4(1000 μhenry) and a capacitor C22 (0.1 μF) which filter and smooth theISET. The filtered ISET is then provided to a resistor R22 (100K ohms),a resistor R39 (121K ohms) and an op amp A3A. Op amp A3 scales the ISET.A Resistor R23 (825K ohms) sets the minimum machine output. A relay CR1shuts down the regulator when the machine is turned off.

The output of op amp A3A is provided through a resistor R40 (43.2K ohms)and a calibrating resistor R70 to the integrating portion of the PIregulator. The integrating portion of the regulator includes and op ampA3C, a resistor R41 (1M ohms), a capacitor C13 (0.33 μF), a resistor R24(82.5K ohms), a capacitor C18 (560 pF), and zener diodes D10 and D11.The components are configured with op amp A3C such that the output of opamp A3C (an error signal) is a signal dependent upon the differencebetween the current reference and the current feedback signals, and theintegral of that difference. The output is then used to trigger theSCR's.

When the error indicates that not enough current is being provided, theSCR's are triggered earlier in their cycles, thus providing more power.When the error signal indicates that too much power is being provided,the SCR's are fired later in their respective cycles.

The portion of controller 103 that provides the CV regulation in thepreferred embodiment is shown in FIG. 5 and it is a PI regulator whichreceives as inputs on pins RC2-4 and RC2-5 the voltages on electrode Eand work W (the output voltage). The output voltage feedback signals areprovided through inductors L51 and L52 (1000 μhenry) and a capacitor C59(0.01 μF) to smooth and filter the feedback signal. The smoothed andfiltered feedback signal is rectified by a full bridge comprised ofdiodes D5-D8. The rectified feedback voltage signal is provided througha filter network comprised of resistors R52 (100K ohms), R53(5.62Kohms), R51(100K ohms), R55(5.62K ohms) and capacitors C51 (0.22 μF) andC52 (0.22 μF).

The signal is then provided to op amp A1A having scaling resistors R54(100K ohms), R56 (100K ohms), R58 (221K ohms) and R57 (221K ohms). Opamp A1A scales the signal and provides it to another op amp AIB havingfeedback resistors R59 (47.5K ohms) and R60 (4.32K ohms). The output ofamp A1B is a scaled feedback signal and is provided through a resistorR61 (10K ohms) to the inverting input of an op amp A1C.

A voltage reference command (VSET) is provided on pin RC2-9 and ispreferably obtained from a potentiometer on the front panel of the powersupply. Of course, other methods such as a remote or digital circuitrymay be used to obtain the voltage reference signal. VSET is thus asignal indicative of the user's desired output voltage when operating inthe CV mode.

VSET is provided through an inductor L5 (1000 μH), which smooths VSET toan op amp A1D, which has scaling resistors R69 (150K ohms) and R68 (100Kohms). The scaled VSET is provided through a resistor R66 (15K ohms) toop amp A1C.

Op amp A1C performs the proportional and integral functions. Thecircuitry associated with op amp A1C, including capacitor C56 (0.001F),capacitor C55 (0.22 μF) resistor R65 (35.7K ohms), and resistor R64(332K ohms), are configured so as to provide the desired PI regulation.The output of op amp A1C is an error signal that is dependent on thedifference, over time, between the voltage reference signal and thevoltage feedback signal. Resistor R62 (61.9K ohms) sets minimum outputvoltage.

The error output of op amp A1C is provided through a diode D9, acapacitor C57 (0.1 μF) and an inductor L6 (1000 μhenry). This erroroutput is provided on pin RC2-6. When the power supply is operated inthe CV mode pin RC2-6 is connected to pin RC2-2, so that ISET is thevoltage regulator output.

Numerous modifications may be made to the present invention which stillfall within the intended scope hereof. For example, other controlcircuitry could be employed, including digital circuitry. A regulatorother than a PI regulator could be used. The regulators could beselected in the alternative, rather than in a cascading arrangement.Other power circuits could be used as well.

Thus, it should be apparent that there has been provided in accordancewith the present invention a method and apparatus for providing sub arcwelding power using a step-up cycloconverter having an AC/DC and CC/CVoutput that fully satisfies the objectives and advantages set forthabove. Although the invention has been described in conjunction withspecific embodiments thereof, it is evident that many alternatives,modifications and variations will be apparent to those skilled in theart. Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. An arc welding powersupply comprising: a step-up cycloconverter having a primary side and asecondary side, and at least one control input on the secondary side; afeedback circuit that provides a signal indicative of the outputvoltage; and a controller coupled to the control input and the feedbackcircuit, wherein the controller controls the output of thecycloconverter in response to the output voltage to provide a constantvoltage output.
 2. The welding power supply of claim 1, furtherincluding a current feedback circuit that provides a signal indicativeof the output current; and wherein the controller is further coupled tothe current feedback circuit, and further wherein the controllercontrols the output of the cycloconverter in response to a userselectable one of output current and output voltage.